The Wiegand protocol is the predominant method by which physical access control card readers communicate with upstream devices such as local controllers, access control panels and host computer systems. Because of the popularity and almost universal support of the Wiegand protocol in access control panels, other devices besides access control readers are also available that support the Wiegand protocol. Such devices include biometric-based devices such as fingerprint, hand-geometry, iris, facial recognition, and vein scan. Other devices that utilize the Wiegand protocol include motion sensors, thermostats and smoke detectors.
Both the electrical and logical aspects of the Wiegand communication protocol are codified in the Security Industry Association (SIA) standard AC-01 entitled “Access Control Standard Protocol for the 26-Bit Wiegand Reader Interface”, the entire contents of which are incorporated herein by this reference in their entirety and will henceforth be referred to as the “SIA standard”.
Subsequent to the issuing of this standard, both the electrical and logical portions of the standard have been used to transmit bit stream messages, often called formats, longer than 26 bits. 35- and 37-bit formats are found widely and the US Government's PIV standard defines some formats of up to 300 bits. The evolution of upstream devices and middleware to use these longer formats has been slow and is still taking place.
Although other methods are utilized for carrying the informational aspects of the Wiegand protocol over communication bearers such as RS-485, F/2F, and various Internet protocols such as TCP/IP and UDP, none has achieved the widespread usage that Wiegand has in the security and access control market segments This is primarily because each manufacturer utilizes their own proprietary protocols even when using standardized communication bearers such as TCP/IP.
The widespread adoption of the Wiegand protocol is due to several advantages with the primary advantages of the Wiegand protocol being that its implementation in devices is very economical and that it allows very long cable runs which, depending on the gage of the wire used, can be as long as 500 feet.
The electrical aspect of the Wiegand protocol uses five wires. Two of these wires are used to provide power to the reader. The remaining three wires are used for data communication and signaling and use the open collector electrical standard, which means that the circuit acts as either an infinite resistance or a short circuit to ground. Typically the upstream device employs a pull-up resistor, which keeps the signal at a high voltage (+5) when it is in the open circuit state. When the signal is asserted, the output is forced to 0 volts. Note that the open state (+5 volts) represents a data value of zero and the asserted state (0 volts) represents the data value of one. This is generally referred to as an “active low” configuration where the active state is the low voltage.
Two of the three data communication and signaling wires are used by the reader to transmit data to an upstream device e.g., control panel, intermediate device, routing device, lock control mechanism, computing platform, host, or the like. These two wires are referred to as DATA0 and DATA1. As the names suggest, the DATA0 signal transmits the “0” bits of the data stream to the upstream device, and the DATA1 signal transmits the “1” bits. FIG. 1 graphically illustrates the representation of a Wiegand data stream for the binary data of “01101”. Each dip in the line represents a change from 5V to 0V, thus communicating a single bit of data of the entire message.
The third data communication and signaling wire is used by the upstream device to signal the reader. This wire is called LEDCTL because it is often used by the upstream device to control a light-emitting diode (LED) in the reader and provides feedback to the card holder.